Effective residue-residue interaction energies have been statistically derived from protein X-ray structures. A lattice-like model is used in which each residue type has a coordination number. If a specific residue has an incompletely filled coordination shell, then it is assumed to be filled with equivalent water molecules. Derived contact energies follow intuition: The most favorably interacting pairs are hydrophobic residues. Howver, those interactions are quite non-specific. More specificity is observed between polar residues. Protein folding schemes based on such energies favoring hydrophobic interactions are being develped.